Metal oxide-hydrogen batteries, such as nickel oxide-hydrogen batteries, have seen use as aircraft starter batteries and in aerospace applications because they are rechargeable, have an extremely long cycle life and provide a uniform output during the entire discharge cycle.
In the typical nickel oxide-hydrogen battery, the battery cells are sealed in an outer pressure vessel that contains pressurized hydrogen gas. On discharge of the battery, the hydrogen gas diffuses through the electrolyte surrounding the catalyst surfaces of the negative plates and becomes disassociated to the monoatomic form. The mono-atomic hydrogen is ionized and combines with hydroxyl ions to form water with an electron being released in the process of forming each hydrogen ion. In addition, hydroxyl ions are formed at the positive electrode by the reaction of water with the available oxygen content of the nickel oxide. As a result of these reactions, an electron current is produced in the exterior circuit.
On recharging, the reaction is reversed, with the recharging being characterized by the regeneration of hydrogen at the negative electrode and the reoxidation of nickel hydroxide at the positive electrode.
Potassium hydroxide is commonly used as the electrolyte in a metal oxide-hydrogen battery. Because of the high wetability of potassium hydroxide, extreme care must be exercised to prevent capillary migration of the potassium hydroxide and bridging between the cell modules and causing a shorting path.
If oxygen produced during the cycling of the metal oxide-hydrogen batteries escapes from the cell module, it can recombine non-uniformly in the various cell modules of the battery. Since the formation of oxygen occurs by consuming water and its recombination with hydrogen at any negative electrode forms water, the net result of non-uniform recombination can lead to drying out of some cell modules which can hinder performance of the battery and ultimately cause its failure.